Coherent recombination of laser beams is a technique used to solve the problem of flux stability limitation of gain materials for the purpose of obtaining a high-power laser source. In particular, high-power fiber laser sources are thus produced. This technique makes it possible to obtain a laser beam of high luminance, but also of high coherence and high optical quality (diffraction limited). Laser sources based on coherent beam recombination thus make it possible to envisage power levels that it would not be possible to obtain from a single fiber, owing to the flux stability limitation. To give an example, an ytterbium-doped polarization-maintaining fiber laser makes it possible to extract 500 W of monomode continuous power. A laser source comprising a bouquet of around one hundred of these fibers, with a coherent recombination system, would allow 50 kW of monomode power to be extracted, which power is impossible to obtain from a single fiber. This shows the great benefit of coherent beam recombination.
The general principle of this coherent recombination technique, in which N elementary laser beams are recombined, is to distribute the necessary amplification over N gain media undergoing spatial monomode propagation. The summation of N coherent beams is carried out, as output from the N gain media. N is chosen to be as large as necessary depending on the intended application. The beams to be recombined may in practice be in the form of a one-dimensional (1D) array or a two-dimensional (2D) array as a P×Q. matrix. In the rest of the description, the case of a 1D array is considered, but everything that is described can apply just as well to a 2D array.
Coherent recombination thus consists in summing N coherent beams of the same polarization in parallel, each amplified by propagation in a gain medium. If the N laser beams that emerge from the N gain media are in phase, they interfere constructively and thus constitute a source having a luminance of N2 times greater than that of an elementary amplifier (i.e. 1 beam and 1 gain medium), while maintaining its beam quality (diffraction limited in the case of monomode fibers for example).
However, the N beams follow different propagation paths, and thus undergo different phase variations. These phase variations are due to index variations of many origins: environmental conditions (temperature, vibrations, mechanical stresses, etc.) passages through pumped gain media, etc. These various phase perturbations vary with time.
Thus, these laser sources require a dynamic control device for controlling the phase of each beam, which allows the phase differences between the various beams to be corrected and cancelled out in real time. This ensures that the laser beam resulting from the recombination is very stable under severe environmental conditions. Furthermore, such a device makes it easier to take into account any missing element.
Dynamically controlling the phase of each beam has other known advantages, such as that of providing a beam scanning function. This is particularly beneficial in optronic applications, such as for example designation, tracking or pointing, or communications in free space.
A diagram showing the principle of a laser source based on coherent beam recombination according to the prior art is illustrated in FIG. 1.
This source comprises N incident laser beams Lij, where j=1 to N, N phase shifters Dj—one per incident laser beam—and N spatial monomode propagation channels gj—one per incident laser beam. In the case of a high-power laser source, these propagation channels are preferably gain media, advantageously fiber amplifiers. The N incident laser beams are spatial monomode beams of the same polarization.
A coherent recombination system 1 receives a beam made up of N laser sub-beams Laj, obtained as output from the N channels gj. It delivers a recombined laser beam fR as output.
The coherent recombination system comprises a device 2 for taking a part of the output beam to a phase-lock device 3. This phase-lock device delivers the feedback signal to each of the phase shifters Dj. The device 2 may for example comprise an array of microlenses in an example of coherent recombination of free-space beams.
The phase-lock device 3 generates feedback signals according to an appropriate feedback control algorithm applied to a tiny portion of the beam taken by the device 2. The device 3 measures, on this tiny portion of the beam taken, the phase differences between the sub-beams Laj. It generates the feedback signals to be applied to the phase shifters Dj based on these measures. Each of the N phase shifters Dj is thus under closed-loop feedback control via a corresponding feedback signal generated by the phase-lock device 3. This signal determines the effectiveness of the coherent recombination system 1 in bringing the N beams Laj at the output of the channels gj into phase.
The invention relates more particularly to the phase-lock device 3. This phase-lock device must meet various constraints, which determine the efficiency of the coherent recombination system 1 in bringing the beams into phase.
A first constraint is the rate of phase correction. This is because gain media, which are preferably fiber gain media (i.e. fiber lasers), are generally very long and very sensitive to environmental perturbations. This imposes a high phase correction rate, typically of the order of several kHz.
A second constraint lies in the measurement to be made for correcting the phase. It is not a question of measuring the aberrations of a single beam, as in other imaging applications, since the coherent recombination system 1 receives N beams. Therefore the phase shifts between each of these N beams have to be measured so as to recombine them efficiently. However, this does not mean analyzing all the aberrations either: since the N beams are all monomode, each is thus virtually diffraction-limited and free of aberration. The phase shifts to be measured and to be corrected thus correspond to zero-order aberrations, that is to say phase pistons, and possibly to 1st-order aberrations, that is to say “tilts”.
It may be considered that each of the beams received is seen by the coherent recombination system as a sub-pupil. Each of the N sub-pupils seen by the coherent recombination system is phase-shifted relative to the other sub-pupils by a constant phase shift, i.e. by a piston, that is to say a zero-order aberration, or by a tilt of the wave surfaces of each of the pupils, that is to say a 1st-order aberration.
A phase-lock device for a beam recombination system must allow these aberrations to be measured and corrected, at the necessary rate, in order to be very effective.
Various systems for measuring and correcting the phase are known.
In a system based on an interferometric analysis method, the phase shift between the sub-pupil of each beam and a reference is measured. This reference may be delivered by one of the N beams, or more simply by an additional, reference beam, which is phase-modulated. This reference beam acts as a local oscillator. If the propagation media are fiber gain media, the reference beam may be brought to the entry of the coherent recombination system via a non-amplifying fiber.
According to this method of analysis, the phase-lock device 3 goes back to the phase difference of each sub-pupil relative to the reference by demodulating the detected signals in phase quadrature.
This method of analysis has the drawback of being complicated to implement for a large number N of beams since, roughly, it consists in producing an interferometer for each sub-pupil. This therefore requires the provision of a detection system for each channel gj with the alignment problems that this entails, in addition to managing an additional reference beam. In practice, this method may be implemented for a small number of beams, typically up to N=4 for example. In a system in which there may be around one hundred beams, this method becomes very tricky to implement.
Another method is known, which uses the phase shifters Dj (the bandwidth of which must be adapted accordingly) for impressing on each of the N beams, in addition to the phase correction, an RF phase modulation (at several tens or hundreds of MHz), i.e. a much more rapid modulation than the frequency of the phase corrections (typically less than a few tens of kHz). This modulation must be different for each beam. Each sub-pupil detected is thus discriminated from the others by a different RF frequency. The detection uses a single detector, typically a photodiode. The associated signal processing makes it possible to go back to the phase difference of each of the detected sub-pupils by analyzing the actual phase shift at each RF frequency. This method, which requires only a single detector, has the advantage of being simpler to implement than the interferometric analysis method. In addition, it does not require a reference beam. On the downside, it requires more complex processing electronics, especially electronics capable of generating N different RF frequencies. However, this is not its main drawback. This is because with such a method, for applications of the high-power laser source of the data transmission type in free-space communication systems, each of the N beams is modulated at an RF frequency, which frequency could lie within the bandwidth of the signals to be transmitted. This is a disadvantage that limits the application options of this method.
Also known are wavefront analyzers, especially phase-shift interferometers, multiple-wave interferometers, and wavefront analyzers of the Shack-Hartmann sensor type. However, these analyzers are ill-suited for phase-shift measurement in a coherent beam recombination system. This is because they are very complicated and difficult to implement. However, above all their correction rate is low, generally less than 1 kHz, thereby precluding a real-time correction at the desired rate. Furthermore, a Shack-Hartmann wavefront sensor does not allow zero-order aberrations (phase pistons) to be measured.